Aquametry is a method of determining the moisture content of solids and liquids and is practiced in numerous industries. For example, the determination of moisture content is important in grain to determine propensity for spoilage, in soil to determine irrigation requirements, in sand to determine proper water proportioning in batching concrete, in paper manufacturing to verify requisite dryness, and in food processing to verify appropriate cooking times. Common venues for moisture determination include laboratories for research and quality control, inside storage bins, and within processing equipment to measure moisture during various processes.
The most common methods of moisture determination include measuring weight loss upon heating, measuring humidity within a cavity containing the sample, and various titration methods. None of these methods, however, are nondestructive, nonintrusive and instantaneous.
Microwave based aquametry methods are categorized as either transmissive or reflective, and are based on the high dielectric constant of water which causes moisture to be both highly absorptive and reflective in the microwave and infrared regions. Commercially available infrared reflectance devices sense only surface moisture. Known commercially available microwave transmission methods are susceptible to interferences due to extraneous reflections and embedded metal in the sample being analyzed, require access to opposite sides of the sample, and are limited to measuring low bulk moisture because of the high absorptivity of water.
U.S. Pat. No. 4,364,008 to Jacques discloses a moisture content analyzer which utilizes the capacitive method of measuring moisture content. The capacitive method entails measuring the increase in electrical capacitance caused by placing the sample within the electric field of a capacitor. In the Jacques patent, an electromagnetic wave created by an oscillating electric field distributes itself so that a standing wave occurs in a resonant cavity. Moisture in the sample interacts with the electromagnetic wave to shift the frequency of the standing wave. The magnitude of the shift is determinative of the amount of moisture in the sample. Being capacitive in nature, the disclosed method requires a larger sample than is necessary for other methods of moisture determination.
U.S. Pat. No. 4,156,843 to Strandberg, Jr. et al. discloses a microwave reflectance based moisture indicator for thin webbed material. The Strandberg device irradiates the sample with an X-band Gunn oscillator and measures the reflected energy with a diode detector. Both the source and the receiver are fitted with horns which are angularly mounted with respect to each other on one side of the moving web.
Rzepecka, et al., "Monitoring of Concrete Curing Process by Microwave Terminal Measurements", IEEE Transactions on Industrial Electronics and Control Instrumentation, Vol. 19, No. 4, November 1972, discloses a method of monitoring concrete curing processes using microwaves. The device described in the article utilizes a single horn serving as both the source and the receiver through use of a directional coupler. The device is used to study dielectric changes in hydrating concrete, but not to quantitatively measure moisture.
Microwave aquametry techniques, particularly transmissive techniques, are adversely affected by numerous reflections from the sample surfaces, the microwave components, and the environment. These multiple reflections give rise to a wave propagation pattern that is extremely complex, varies greatly with even small variations in sample size and component positioning, and is impractical to predict.
Accordingly, there is a need for a simple and inexpensive microwave moisture analyzer which operates in the reflective mode and which requires no receiver or transmitter horns or antennae. The present invention addresses this need by providing a microwave moisture analyzer which operates in a nondestructive and noninvasive manner and which provides an instantaneous indication of the moisture content of a sample being analyzed.